For many years, image reproduction technology has relied on, inter alia, first producing an image on a paper original and reproducing the original image using a xerographic based process. With the advent and increasingly widespread use of personal computers, such images increasingly contain computer generated graphics, such as pictures, charts, graphs and the like of one form or another. In forming an original depiction of such an image, a desired graphical image is often generated onto a sheet of paper or other suitable medium using an output device, such as a pen plotter or the like. This original depiction is then xerographically reproduced a desired number of times. Xerographic reproduction generally involves placing a paper original face down on a platen of a xerographic copier and then directing light onto the image depicted thereon at an appropriate angle such that light reflected therefrom will strike a surface of an appropriately charged moving photoconductive drum or belt (henceforth referred to as a photoconductor) as it passes through an internal exposure station within the copier. The reflected light, in turn, locally discharges the surface of the photoconductor such that a resulting electro-static charge pattern appearing thereon substantially matches the local visual reflectance characteristics that appear in the image. When the rotating photoconductor reaches an internal toning station within the copier, toner typically in the form of a powder is automatically applied to the photoconductor. The toner adheres to those portions of the surface of the photoconductor that remain charged. As the photoconductor continues to rotate, a sheet of paper is subsequently pressed against the rotating drum at a transfer station internal to the copier. An opposite charge is applied to the paper in order to transfer the toner pattern from the photoconductor to the paper. Thereafter, the paper is separated from the photoconductor typically through application of an appropriate charge thereto. Thereafter, the "toned" image is permanently fixed onto the paper at a so-called "fusing" station within the copier whereat the paper is passed between two heated rollers which melt the toner and fuse it into the paper.
Owing to the relatively large sized optical components typically used in a xerographic copier and the number and/or size of the necessary optical transmission paths internal to the copier, xerographic copiers tend to be physically large and rather bulky. Moreover, a user often wastes a significant amount of time by first employing a pen plotter or other similar device to generate an original image and then manually reproducing the image using a xerographic copier -- the latter task includes bringing the original to a copier, waiting for the copier to generate the desired number of copies and then returning with the copies.
Therefore, in an effort to substantially increase the speed at which multiple copies of a image can be produced while reducing the size of an output device that produces these images, the art has turned to electronic imaging techniques. Generally, these techniques convert digital data directly into an image at a sufficiently high quality to rival present optical image reproduction techniques. In one such electronic imaging technique, digitized binary, gray scale or color image data provided by a computer or similar device, rather than light reflected off a paper original, is used to repeatedly discharge a photoconductor that through one or more separate toning passes respectively generates either a black and white or color image at a resolution that favorably compares with that produced by a optical xerographic copier. Specifically, the digital data is used, through appropriate driving circuitry, to energize individual diodes that exist within a linear array of light emitting diodes (LEDs) that collectively form a printhead. In response to the drive signals, the individual diodes generate light energy that when passed through a fiber optic lens assembly onto the surface of a moving photoconductor is sufficiently intense to locally discharge the surface of the photoconductor and establish a charge pattern thereon that mirrors a desired visual graphical pattern. To make multiple copies, this electro-optical imaging process is then repeated as often as necessary to directly generate the desired number of copies. Moreover, if an image that has been previously generated on paper or another medium is to be copied, that image can be read and digitized using a facsimile type scanner, stored within a digital memory circuit and subsequently and repeatedly printed using such a digital image printer to provide one or more copies.
To provide light energy that closely matches the spectral sensitivity of the photoconductor, gallium arsenide diodes that produce red light are used typically within the printhead. Unfortunately, present gallium arsenide fabrication technology suffers from a drawback that severely complicates the assembly of LED printheads.
Specifically, LED printheads generally require a single relatively long row, generally 11" (approximately 28 cm) or greater, of separate light emitting sites. Furthermore, to provide an appropriate level of detail in an output image which rivals that produced by xerographic or other image reproduction methods, LED printheads typically need a minimum resolution of 400 light emitting sites, i.e. individual LEDs, per inch (approximately 158 LEDs/cm). This necessitates that an 11" printhead must have at least approximately 4400 separate diodes aligned in a single row with a resulting 2.5.multidot.10.sup.-6 " (63.5 .mu.m) center-to-center spacing between any two adjacent diodes. Unfortunately, current gallium arsenide fabrication methods have not reached the level of sophistication needed to produce semiconductor wafers in excess of typically 3" (approximately 7.6 cm) in diameter. Accordingly, the relatively small size of these wafers prevents a single 11" row of gallium arsenide LEDs from being fabricated on a single substrate. Hence, the art has turned to fabricating LED printheads using a sequence of individual arrays of gallium arsenide LEDs that are arranged in an abutting end-to-end fashion to form a single common line of closely spaced light emitting sites, in which each array contains multiple, e.g. 128, LEDs arranged along a single row. To ensure that the photoconductor will be uniformly illuminated along the entire width of the printhead, thereby ensuring to the extent possible that no artifacts due to uneven illumination will be imparted into an output image produced therewith, the individual LED arrays must be positioned on the printhead within extremely fine tolerances with respect to each other not only two-dimensionally across a common transverse axis on the printhead but also elevationally across the entire printhead, the latter ensuring that the printhead possesses a sufficient degree of mechanical flatness.
With this overall approach to implementing an LED printhead in mind, various specific techniques for actually implementing this approach are disclosed in the art. However, each of these techniques experiences various deficiencies that limit its use.
One technique, hereinafter referred to as the "ceramic substrate" technique for reasons that will become clear below and typified by the disclosure in U.S. Pat. No. 4,734,714 (issued to Takasu et al on Mar. 29, 1988), involves an LED printhead in which individual LED arrays, each having 96 light emitting sites, are each positioned on a relatively wide thick film conductive strip located along a central transverse axis of a surface of a fired alumina ceramic substrate. Staggered anode connections for all the diodes and associated metallized pads ("anode pads") therefor appear on the top of each array. The cathodes of all the individual LEDs within any array are internally connected to a common gold electrode on the reverse side of the array that abuts against the conductive strip. Each individual diode is approximately 0.04" by 0.31" (1 millimeter by 8 millimeter) and is arranged within an array at a center-to-center spacing of approximately 33.3.multidot.10.sup.-6 " (84.5 .mu.m) between adjacent diodes. A pair of drive driving elements is associated with each individual array. The two elements that form any such pair are mounted directly to the substrate and on opposite sides of and generally perpendicular to the corresponding array. These driving elements contain appropriate shift registers and LED drive circuits. For any one array, one driving element in the pair associated therewith (situated in the so-called even half of the printhead) controls even number LEDs in that array, while the other driving element in the pair (situated in the so-called odd half of the printhead) controls the odd number LEDs in that array.
In this specific LED printhead, LED drive signals are routed from a connector situated near an edge of the substrate to various signal processing and line driver integrated circuits that are also mounted on the substrate. The output signals produced by the line drivers are applied through appropriate metallized busses situated on the surface of the substrate to drive driving elements for either the odd or even half of the printhead. The pitch of the output terminations of the driving elements is significantly greater than the narrow center-to-center pitch of the anode pads for the individual LEDs. Accordingly, for each driving element, a pattern of metallized interconnection leads ("interconnects") having a pitch that matches that of the driving element terminations is also fabricated on the surface of the substrate. These interconnects have pads at one end that are linearly aligned for connection to appropriate terminations of a driving element and have staggered metallized pads at the other end thereof for connection to corresponding individual metallized anode pads of the LEDs. One end of every interconnect is connected through a wire bond using relatively fine wire to an individual anode pad of an LED; while the other end of every metallized lead is connected through another wire bond to a corresponding drive module termination. Wire bonds, again with relatively fine wire, are also used to connect appropriate line driver terminations to the metallized busses. The metallized busses and interconnects are collectively formed by placing a gold thin film onto the alumina substrate followed by one or more separate conductive thick film and interspersed dielectric layers to form, where necessary, a multi-layered metallized pattern on the surface of the substrate. Flexible circuitry is used to route power from external circuit connections to multiple metallized leads situated on either side of the printhead. The substrate itself is affixed to a relatively large heatsink.
This specific technique for implementing an LED printhead is plagued by a number of serious deficiencies. First, long dimensionally accurate fired ceramic substrates that maintain flatness within acceptable tolerances across their entire length, such as 25 .mu.m over 12" (approximately 30.48 cm), have proven to be extremely difficult to manufacture in large quantities. Inasmuch as low yields of acceptable substrates typically occur, each resulting substrate tends to be very expensive. Second, this approach requires a large number of wire bonds, typically in excess of 10,000 which are expensive and time-consuming to provide and also tend to reduce reliability of the printhead. Third, since the individual LED arrays are mounted through a thick film conductor to the actual ceramic substrate which, in turn, is mounted to a heatsink, a relatively high thermal resistance exists for heat dissipated from each diode, which, in turn, during sustained operation of the printhead raises the temperature and the failure rate of the individual LEDs therein. Fourth, inasmuch as a relatively high current is needed to drive the printhead, this current causes voltage drops to appear across the flexible circuits used to distribute power. Specifically, each diode in an operating printhead draws an average current of approximately 8 mA. Assuming every diode in the printhead is simultaneously energized, then an entire printhead containing 5000 such diodes draws approximately 40 amperes during the 50% on time of the duty cycle associated with all the diodes, with half of this current being distributed through flexible circuitry to the each of the even and odd halves of the printhead. Owing to the relatively small cross-section of the copper conductor(s) contained in the flexible circuitry used to route power to each half of the printhead, an appreciable voltage drop appears across this circuitry particularly when all or most of the diodes are energized. This voltage drop increasingly lowers the drive current available to power the diodes that are located at increasing distances down this circuitry and along the printhead such that "current starvation" is increasingly likely to occur for these diodes. Consequently, the printhead disadvantageously produces a non-uniform optical output across its length. Fifth, since all the driving elements and LED arrays are mounted to a common substrate, any subsequent failure in any of these driving elements or an LED array itself necessitates that manual repair techniques be used to replace a failed component, i.e. a driving element or an LED array, without damaging any of the other components on the substrate. This is generally an extremely difficult and expensive task. Moreover, since not every repair is successful or can be economically accomplished, the affected printheads including the large ceramic substrate and all the components mounted thereto, which are collectively quite expensive, are merely scrapped resulting in significant economic waste. Furthermore, if a driving element failed, all the relatively fine wire bonds connected to this driving element need to be manually removed, the driving element manually replaced and the bonds manually re-attached to a replacement drive module. This procedure is not only tedious, even when performed by skilled labor, but also the manual nature of this procedure renders it unsuitable for use of a mass production manufacturing environment.
In an effort to surmount these deficiencies associated with the "ceramic substrate" technique, the art has turned to another technique, hereinafter referred to as the "multi-module distribution board" technique for reasons that will become clear below, for fabricating LED printheads. Here, the printhead contains an assembly having a number of modules which are all mounted, typically using a conductive adhesive layer, to a metallic, typically stainless steel, support plate in an abutting horizontally aligned orientation with the support plate, in turn, being mounted to an aluminum heatsink. Each module has a metallic base plate ("tile") that is typically rectangular in shape with a vertical dimension that is somewhat larger than its horizontal dimension. The flatness of each of these metallic tiles can be much more easily maintained to the needed tolerance than can that of a large ceramic substrate.
Specifically, through the "multi-module distribution board" technique, an assembly of one or more LED arrays, illustratively three, is mounted onto a tile and located along a central transverse axis thereof with corresponding drive circuits situated on the tile close to and on opposing sides of each array and interconnected thereto through wire bonds, here at a relatively narrow pitch and using relatively fine wire. A ceramic or printed circuit spreader board is also situated on the tile and is located beyond and on either side of the drive circuits. A row of metallized fingers is typically located along a horizontal edge of the spreader board that is to be situated farthest from the drive circuits. The spreader boards are suitably dimensioned and appropriately situated on each tile such that a relatively small gap exists between one horizontal edge of the spreader board and the drive circuits while the other edge containing the metallized fingers overlays and is generally aligned with a corresponding horizontal edge, i.e. either the top or bottom edge, of the tile. Each spreader board contains a multi-layer metallized wiring pattern for interconnecting appropriate drive terminations to the appropriate metallized fingers. This wiring pattern matches the relatively narrow pitch of the drive terminations to a relatively broad pitch of the fingers and distributes appropriate LED drive signals, such as clocks and power, as input to the proper drive terminations. Metallized leads on the spreader board are connected by wire bonds, at the narrow pitch and again with using relatively fine wire, to appropriate drive terminations. In some current printhead implementations, each spreader board is typically a co-fired ceramic multi-layer thick film hybrid board which uses gold thin film layers for metallized bond pads and a relatively resistive conductor, such as tungsten, for the conductive layers. Dielectric layers are included in the spreader board such that a cross-over pattern of perpendicularly oriented conductive layers interconnected with appropriate feedthroughs (also known as "vias") is formed on each spreader board. This arrangement also includes a large conventional multi-layer laminate rectangular circuit board with a centrally located rectangular cut-out which is appropriately sized to substantially encircle the entire assembly of modules. This multi-layer board, henceforth referred to as the "distribution" board, contains metallized busses, typically 25 or more, to distribute power and drive signals, the latter being produced by various signal processing and line driver integrated circuits located on the distribution board, to the proper fingers of each spreader board. Wire bonds, though here with a relatively wide pitch and a relatively large diameter wire, connect the appropriate busses on the distribution board with corresponding fingers on each spreader board. Power and incoming data and clock signals are supplied to the printhead through appropriate connectors typically situated on the distribution board and located relatively close to an edge thereof.
While the "multi-module distribution board" technique eliminates various drawbacks associated with the "ceramic substrate" technique, it nevertheless presents other drawbacks. Since the LED arrays are directly mounted through a metallic path to the heatsink, heat is more readily dissipated therefrom than in the "ceramic substrate" technique thereby beneficially lowering the failure rate of the LEDs. Furthermore, since each individual module can be fully tested after its assembly but prior to its being mounted to the support plate, the need to repair completed printheads substantially decreases. Moreover, whenever such a repair is needed, a complete module can be readily removed from the printhead and a replacement installed thereon. Inasmuch as this repair necessitates removing a small number of wire bonds that occur at a relatively wide pitch between the fingers on that module and the distribution board, installing a new module and then replacing these wide-pitched bonds, the cost and tedium associated with this repair advantageously is significantly less than that associated with replacing a failed component located on an LED printhead implemented using the "ceramic substrate" technique. However, though the "multi-module distribution board" technique eliminates the need to use a large ceramic substrate along with its attendant high cost, the "distribution" board is still expensive though less than the ceramic substrate. For example, wire bondable gold is generally used in a wire bond layer within the distribution board which increases its cost. In addition, if ceramic spreader boards are used, these spreader boards themselves tend to be costly. Nevertheless, the combined cost of a distribution board and all the required attendant spreader boards is often appreciably less than the cost of a large ceramic substrate. Second, the signal distribution lines running between and among both the distribution board and the spreader boards present complex impedance values, typically containing resistance, inductance and a significant amount of capacitance, that due to inherent charge and discharge times associated therewith limit the speed of clock and data signals that can propagate down the printhead and hence limit the speed at which the printhead can perform. Third, the cross-sectional area of the metallized busses situated on the distribution board that carry power to each half of the printhead still tends to be insufficient to eliminate an appreciable voltage drop that appears therealong during operation of the printhead. This voltage drop reduces the available drive current to the individual LEDs situated at increasing distances down from the printhead and, in turn, through current starvation causes a non-uniform optical output to appear along the printhead. Fourth, the distribution board is relatively large which disadvantageously increases the overall physical size of the printhead and any image printer that employs it.
Therefore, a need exists in the art for a printhead, such as an LED printhead, that tends to be smaller, and is simpler and less expensive to implement than such printheads known in the art. Moreover, the resulting LED printhead should provide a more uniform light output across its entire length than currently available printheads; operate at increased speeds than those associated with currently available printheads, particularly printheads implemented using the "multi-module distribution board" technique; have a relatively low thermal failure rate, and be relatively easy and inexpensive to repair. Such a resulting printhead will advantageously facilitate the evolution of relatively small and inexpensive electronic image printers.